Commit 57a1b089 authored Jul 17, 2019 by Sean Paul

drm: Make the bw/link rate calculations more forgiving



Although the DisplayPort spec explicitly calls out the 1.62/2.7/5.4/8.1
link rates, the value of LINK_BW_SET is calculated.  The DisplayPort
spec says "Main-Link Bandwidth Setting = Value x 0.27Gbps/lane".

A bridge that we're looking to upstream uses 6.75Gbps rate (value 0x19)
[1], and that precludes it from using these functions.

This 6.75Gbps rate is defined in the spec as (credit to Ville for posting this):
  A MyDP Source device, upon reading the MAX_LINK_RATE register of the
  downstream DPRX programmed to 19h (which can be the case only for a
  MyDP-to-Legacy or MyDP-to-DP lane count converter) can program the
  LINK_BW_SET register (DPCD Address 00100h) to 19h to enable 6.75Gbps/lane."

So to avoid failing on legitimate rates in the future, this patch calculates thevalues according to spec instead of restricting these values to one of the
DP_LINK_BW_* #defines.

No functional change for the well-defined values, but we lose the
warning (and return the correct value) for ill-defined bw values.

Signed-off-by: Sean Paul <seanpaul@chromium.org>

[1] https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/1689251/2/drivers/gpu/drm/bridge/analogix/anx7625.c#636



Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190717160148.256826-1-sean@poorly.run

parent 9aef5867

drivers/gpu/drm/drm_dp_helper.c

+4 −27

Original line number	Diff line number	Diff line
		@@ -152,38 +152,15 @@ EXPORT_SYMBOL(drm_dp_link_train_channel_eq_delay);

		u8 drm_dp_link_rate_to_bw_code(int link_rate)
		{
		switch (link_rate) {
		default:
		WARN(1, "unknown DP link rate %d, using %x\n", link_rate,
		DP_LINK_BW_1_62);
		/* fall through */
		case 162000:
		return DP_LINK_BW_1_62;
		case 270000:
		return DP_LINK_BW_2_7;
		case 540000:
		return DP_LINK_BW_5_4;
		case 810000:
		return DP_LINK_BW_8_1;
		}
		/* Spec says link_bw = link_rate / 0.27Gbps */
		return link_rate / 27000;
		}
		EXPORT_SYMBOL(drm_dp_link_rate_to_bw_code);

		int drm_dp_bw_code_to_link_rate(u8 link_bw)
		{
		switch (link_bw) {
		default:
		WARN(1, "unknown DP link BW code %x, using 162000\n", link_bw);
		/* fall through */
		case DP_LINK_BW_1_62:
		return 162000;
		case DP_LINK_BW_2_7:
		return 270000;
		case DP_LINK_BW_5_4:
		return 540000;
		case DP_LINK_BW_8_1:
		return 810000;
		}
		/* Spec says link_rate = link_bw * 0.27Gbps */
		return link_bw * 27000;
		}
		EXPORT_SYMBOL(drm_dp_bw_code_to_link_rate);

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